Gigahertz Peaked Spectrum (GPS) sources are characterised by a convex shaped radio spectrum peaking at about 1 GHz in frequency (O'Dea 1998). The existence of spectral turnovers in these objects implies that their radio emission is confined to very compact regions. Indeed, VLBI observations show that their radio structures are in general smaller than a few hundred parsecs (Stanghellini et al. 1997). The morphologies of GPS sources optically identified with galaxies are typically dominated by two components which are more or less equal in flux density and spectral index. Since sometimes a very compact flat spectrum component can be seen in the center, characteristic of a core, the two dominant outer components are in general interpreted as the hot-spots/mini-lobes. The two-sided morphology of these objects is very distinctive compared to that of compact radio sources in general. When selected from VLBI surveys, they are treated as a separate class (Wilkinson et al. 1994), and named Compact Symmetric Objects (CSO). The overlap between CSO and GPS galaxies is large and it can be assumed they form one and the same class of object. However, the overlap is not complete: orientation effects can alter both the morphology and radio spectrum in such way that the objects are not classified as CSO or GPS respectively (Snellen et al. 1998a). Furthermore, it has been claimed that some CSO do not exhibit a spectral turnover at low frequencies. If true, this would make the young nature of these particular CSOs less likely, since it implies the existence of substantial large scale radio emission. GPS sources identified with quasars, which are mostly found at high redshifts, have core-jet morphologies in general and may not be physically related to the CSO/GPS galaxies (Stanghellini et al. 1997, Snellen et al. 1998b).
Since the initial discovery of Gigahertz Peaked Spectrum (GPS) sources, it has been speculated that these are young objects (Shklovsky 1965, Blake 1970), but only recently, compelling evidence in favour of this hypothesis has been given. The strongest evidence comes from measurements of the separation velocities of the hot-spots in several GPS sources, implying dynamic ages of typically 103 years (see Conway, this volume). Furthermore, detailed measurements of the spectral ages of the somewhat larger Compact Steep Spectrum (CSS) sources indicate spectral ages in the range of 103 - 105 years (see Murgia, this volume). The alternative hypothesis that GPS and CSS sources are old objects situated in a very dense environment impeding the outward motion of the jet is also less likely, since no evidence for any difference between the environments of GPS, CSS and large size radio sources has been found.
In this paper, we discuss results on the early evolution of radio sources from the investigation of three samples of faint and bright GPS and CSS galaxies: 1) The faint GPS sample selected from the WENSS survey (Snellen et al. 1998b, Snellen et al. 1998c, Snellen et al. 1999), 2) the bright GPS sample from Stanghellini et al. (1998), and 3) the CSS sample selected by Fanti et al. (1990).